Question about SDI?

SDI gives you +75% chance to intercept the Nukes(ICBM or Tactical Nuke). If I understand correctly then in average 3/4 Nukes should be intercepted. Now I had war, SDI built but Zara Yacob somehow exploded 4 nukes in a row in my territory and nothing has intercepted. It may be very bad luck but it has chance of 1/64. 1/4*1/4*1/4*1/4. But in my recent games it also takes down less than 75%.. even far less than 50%.

Do I miss something?

I've won with 2.9% odds in an attack, so it is possible.

Tactical nukes have a 50% chance to evade interception.

Ok, then you have only 3/8 chance to intercept Tactical Nuke. It makes things little clearer. 5/8*5/8*5/8*5/8 = 625/4096 ~ 15%. So it was still very good luck for Zara Yacob to get 4 Tactical Nukes through.